In steam turbine systems, piping configuring feedwater and condensate systems is typically made of carbon steel or copper. However, the rate of corrosion is higher in carbon steel at lower pH (acidic) and in copper at higher pH (alkaline). Thus, to prevent corrosion in the feedwater and condensate systems, for example, the water properties must be adjusted to a pH of approximately 9.2 (weak alkalinity) by injecting chemicals into the feedwater flowing through the feedwater system. Specifically, ammonia (NH3) or hydrazine (N2H4) are added to adjust the pH. Furthermore, hydrazine can be added to remove dissolved oxygen in the feedwater which is also a cause of corrosion.
However, due to various factors, air may remain in chemical injection piping for injecting the chemicals into the piping of the feedwater system. When the chemicals are injected into the piping of the feedwater system, the air remaining in the chemical injection piping acts like a damper and can prevent the chemicals from being sufficiently injected. Thus, there is a demand for technology for removing such remaining air.
Examples of such technology include the chemical injection device of Patent Document 1. The chemical injection device of Patent Document 1 includes piping through which main fluid (water) flows, a chemical injection flow path through which the chemicals injected into the piping flows, a main fluid flow meter that detects the flow rate of the main fluid and emits a flow rate signal, a high flow rate pump with a large discharge capacity, a low flow rate pump with a small discharge capacity, and a control unit configured to receive the flow rate signal emitted from the main fluid flow meter and send a flow rate signal to the two pumps. This chemical injection device injects chemicals into the main fluid, switching between the two pumps depending upon whether the main fluid flow rate is high or low.
Additionally, this chemical injection device includes a return flow path for removing air. The return flow path extends from a halfway point of the chemical supply flow path, on which the high flow rate pump and the low flow rate pump are disposed, and returns to a chemical tank that stores the chemicals. This return flow path completes a circulation path connecting one of the pumps and the chemical tank to each other. In other words, the chemical injection device removes air in the flow path by circulating the chemicals in the circulation path, prior to switching between the two pumps.